If you do the canal to Marsden you’ll be climbing over Standedge, so will come into the top end of Saddleworth.
You can do either off road or on road from the top of the hill.
Off road is slightly tricky, not bad, but a bit tricky;
Drop down on the road and then look for the track on the left just after Brun Clough reservoir. This the top of Boat lane which used to be where they took the horses over when the canal boats got legged through the tunnel. It comes down to a couple of houses at Brun Lane, then drops to the Diggle Hotel. Turn right over the railway, then left onto the canal. Follow the canal to Uppermill.
This canal goes all the way to Ashton-U-Lyne, to Portland basin. You can then pick up the canal from Ashton to Manchester which runs in past the Man City Eithiad stadium.
Road wise, you’ll end up on big roads anyway, but from Marsden I’d go down the valley to Uppermill, then left through Greenfield, then right through bottom Mossley, Stalybridge, Ashton, then A662 or A635 into Manchester.
Possible alternative; do the ride down to Uppermill, go in the pub, get the train into Manchester from Greenfield station at 10 minutes to the hour.
Shout out for pub recommendations….